Internally fused electric heater



Aug. 2, 1960 L. E. ROOK INTERNALLY FUSED ELECTRIC HEATER Filed June 9, 1958 INVENTOR.

Lawrence E. Rook His AHorney i lfi e INTERNALLY FUSED ELECTRIC HEATER Lawrence E. Rook, Shelbyville, Ind., assignor to General I Electric Company, a corporation of New York Filed June 9, 1958, Ser. No. 740,940

, Claims. (Cl. 219-19) This invention relates to a novel and improved construction for an electric resistance heater of the sheathed type.

This invention is specifically concerned with cartridge type sheathed heaters, such as are used in soldering irons. A known construction for such a heater comprises a core of electrically insulated material on which is wound a resistance wire heating element. The core and element are assembled substantially coaxially within a tubular metal sheath with the heater being radially spaced from and electrically insulated from the sheath. A pair of terminals are provided at one end of the heater for connection of the heating element to a source of electric power. It is a known practice to lead the wire forming the heating element from the end of the element opposite the terminals through the core to one of the terminals. It is further known practice to lead the wire from the other end of the heating element adjacent the terminals inwardly of the core through a radial slot therein for connection with the other terminal which registers with the radial slot in the core. In a known construction for a soldering iron the heater is engaged within a tubular copper tip holder on one end of which is mounted a soldering iron tip. The tip holder is in turn engaged Within an elongated metal tube forming the shank of the iron.

In a soldering iron using a cartridge heater of the type described, when the sheath of the heater is grounded, such as through the tip holder and shank to one side of the power suppl and when the terminal registering with the radial slot in the core is connected to the opposite side of the power supply, if a short circuit should occur between the sheath and a portion of the heating element adjacent the radial slot in the core, there will be provided a low resistance current path between the sheath and the ungrounded terminal registering with the radial slot. The resulting high current between the ungrounded terminal and the sheath may result in the vaporizing of the portion of the wire element extending through the radial slot in the core with a resulting arcing through the radial slot and between the sheath and ungrounded terminal. Inasmuch as the terminals are normally of greater cross sectional areas than the wire forming the heater element, the ungrounded terminal is able to carry sufficient current to sustain the are long enough for the arc to burn through the sheath. The are may then burn through the tip holder and shank or if there is a spacing between any of the sheath, tip holder or shank, metal vaporized by the arc may provide a low resistance path to the next adjacent member so that an arc may be provided between the shell and ungrounded terminal with a resulting burn through of the shell and a possible spraying of molten metal about the vicinity of the iron. While such an occurrence is not frequent, it has been found to occur, primarily as the result of improper maintenance of the heater and extreme use of the heater under adverse conditions.

It is the primary object of this invention to provide a metal sheathed electric resistance heater of a novel and cited S ates Patent Orifice 2,947,842 Patented Aug. 2, 1960 improved construction which will, in the event of a short circuit between the heater element and sheath, preclude the forming of an are between either terminal of the heater and the metal sheath of the heater.

In accordance with one aspect of my invention, I provide a sheathed electric resistance heater of the cartridge type comprising a helical resistance Wire element wound upon a core with both ends of the wire forming the heater element being directed longitudinally of and within the core before being connected, respectively, to a pair of terminals both located at the same end of the heater. Both terminals are completely electrically isolated from the heating element in a direction radially of the element so as to eliminate any short open passage between a terminal and the heating element. The portions of the heating element wire extending longitudinally of and within the core provide fuses internally of the heater between both terminals and the respective ends of the heating element so that in the event of a short circuit between the heating element and sheath, particularly in an area adjacent an end of the element, the high current which will be drawn through the fuse Wires will cause the wires to open and extinguish any pilot are which might have been formed between the heating element and sheath before the arc can grow s'ufiiciently to burn through the sheath.

A more detailed understanding of my invention may be had by reference to the accompanying drawing in which:

Fig. l is a side view of an exemplary soldering iron partly cut away to reveal a cartridge heater of a type With which my invention is concerned;

Fig. 2 is an enlarged perspective view, partly in section, of a cartridge heater such as shown in Fig. 1 constructed in accordance with my invention;

Fig. 3 is a fragmentary perspective view, partly in section, of one end portion of the core and heating element of the heater of Fig. 2; and

Fig. 4 is an end view of the core and heating element of Fig. 3.

With reference to Fig. l of the drawing, an exemplary soldering iron, of a type which a cartridge heater constructed in accordance with my invention might be utilized, comprises a handle 10 from one end of which extends a tubular metal shank 12. A generally tubular metal tip holder 14 is engaged within the outer end of the shank and threadably receives a tip 16 in its outer end. A cartridge heater 18 is engaged in the tip holder rearwardly of the tip 16 and a pair of electrical leads 20, only one of which is shown, extend from the inner end of the heater through the shank and are connected in a suitable manner, not shown, within the handle 12 to a power cable 22 extending into the handle.

With particular reference to Figs. 2 to 4, the cartridge heater 18 comprises an outer tubular metal shell or sheath 24 closed at one end and in which is received an electrically non-conducting core or former 25. A helical resistance wire heating element 28 is wound upon the core, and the core and heating element are assembled substantially coaxially within the sheath in radially spaced relation with an electrically insulating disc 30 disposed between the closed end of the sheath and the inner end 31 of the core. Insulating material 32, such as compacted magnesium oxide, fills the annular space between the heating element and sheath to electrically insulate the element from the sheath. The open end of the sheath is closed by ceramic head 34 sealing this end of the heater. An insulating disc 36 is disposed between the ceramic head 34 and the adjacent outer end 35 of the core.

In accordance with the illustrated embodiment of the invention, the core 26 is provided with three internal passages 38, 39, and 40 extending longitudinally through the core in radially or laterally spaced relation. A slot 42 extends laterally of the inner end 31 of the core and joins one end of the passages 38 and 39. The end portion 45 of the wire forming the heating element extends from the end of the element next adjacent the outer end 35 of the core inwardly of the core through a radial slot or passage 46 registering with the passage 38. The end portion 45 then extends through the passage 38, slot 42, and passage 39 and terminates adjacent the outer end 35 of the core. A terminal 48 is engaged in the end of passage 39 next adjacent the outer end 35 of the core and is connected to the terminal end of the end portion 45 of the resistance wire and to a lead wire 50 extending outwardly of the heater through the ceramic head 34. The terminal 48 and lead wire 50 are completely insulated from the sheath 24 in a direction radially of the sheath by the core 26, insulating disc 36, and ceramic head 34. The other end portion 52 of the resistance Wire extends inwardly of the core through a slot 44 extending laterally of the inner end 31 of the core and registering with the passage 40. The wire 52 then extends longitudinally of the passage 40 and terminates adjacent the outer end 35 of the core. A

jacent said other end of said heating element, the other end of said wire extending from said other end of said heating element radially inwardly thereof and then longitudinally of and within said heating element toward said one end thereof and then being redirected toward said other end of said heating element and terminating adjacent said other end of said heating element, the portions of terminal 54 is engaged in the end of the passage 40 adjacent the outer end 35 of the core and is connected to the terminal end of the wire 52 and to a lead 56 extending externally of the heater through the ceramic head 34. The terminal 54 and lead 56 are completely insulated from the sheath in the same manner as the terminal 48 and lead 50.

In the operation of a heater constructed as described, it a short circuit should develop between the heating element and the sheath, for example at the point where the end 45 of the resistance wire enters the core, and if the lead 50 and terminal 48 are at that time connected to opposite sides of the power source, an arc might develop between the sheath and heating element due to vaporizing of a portion of the heating element. However, the high current necessary to sustain the arc must be drawn through the end portion 45 of the resistance wire disposed in the passages 33 and 39, and such a high current will be suificient to melt a portion of the wire in the passages resulting in an open circuit and extinguishment of the pilot arc before the arc can grow sufficiently to burn through the sheath. If an arc should occur between the sheath and the end of the heating element where the wire 52 enters the core, the same result would be achieved for a corresponding reason. The enclosing of the ends of the heater element wire in longitudinal passages within the core permits the complete isolation of the terminals from the sheath and eliminates any short direct open passage in the core between a terminal and an adjacent end of the heater element through which an arc could play directly to the terminal.

While my invention has been specifically described in terms of a cartridge type sheath heater, it should of course be apparent that it has utility and would be advantageous when used with other types of sheath heaters and regardless of whether the leads to the heater extend from the same or opposite ends of the heater. Further, it will be apparent to those skilled in the art that the specific construction shown and described may be modified or altered without departing from the scope of my invention. Accordingly, the foregoing description should be taken only in an illustrative sense and not as limiting my invention, it being my intention that the invention shall be limited only by the terms of the appended claims which shall include within their scope all structure which logically falls within the language of the claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A sheathed electric resistance heater comprising a helical resistance wire heating element, one end of the wire forming said heating element extending from one end of said heating element radially inwardly of and then longitudinally of and within said heating element toward the other end of said heating element and terminating adsaid Wire extending longitudinally of and within said heating element being radially spaced apart in electrically insulated relation to each other and to the remainder of said heating element, and a pair of terminals disposed adjacent said other end of said heating element in radially inwardly spaced relationthereto and connected respectively to the terminal ends of said wire.

'2. A sheathed electric resistance heating element comprising an elongated core of electrically insulative material having three passages extending longitudinally therethrough in laterally spaced relation and having a slot adjacent one end extending laterally of the core and joining two passages of said three passages; a pair of terminals extending, respectively, into one of said two passages and into the third passage of said three passages at the ends of the passages next adjacent the other end of the core; and a resistance wire heating element disposed about the core with one end of the wire forming the element extending from the end of the heating element adjacent said one end of the core into and through said third passage toward said other end of the core, and being connected at its terminal end to the terminal extending into said third passage; the other end of the wire forming said heating element extending from the end of the element adjacent said other end of the core into the other of said two passages and through said other of said two passages toward said one end of the core, then through said slot and into said one of said two passages and toward said other end of the core and being connected at its terminal end to the terminal received in said one passage.

3. A sheathed electric heater comprising a tubular metal sheath, a resistance wire heating element disposed within the sheath in electrically insulated relation thereto, a pair of conductors of low electrical resistance forming terminals disposed within the sheath, radially im pervious solid insulation for electrically isolating both of said terminals from said heating element and said sheath in a radial direction, and a pair of fusible elements respectively connecting said terminals to said heating element, said fusible elements being electrically isolated from said sheath by solid insulation means for a substantial distance from the ends of said terminals respectively to form a long are path between each of said terminals and said sheath in the event of an arc therebetween.

4. A sheathed electric heater comprising a tubular metal sheath, a resistance wire heating element disposed within the sheath and electrically isolated therefrom, a pair of conductors of low electrical resistance forming terminals disposed within the sheath, radially impervious solid insulation means for electrically isolating both of said terminals from said heating element and said sheath in a radial direction, and a pair of fusible elements respectively connecting said terminals to said heating element, each of said fusible elements being electrically isolated from said sheath by solid insulation means for a substantial distance from the end of its respective terminal and having a substantially higher electrical resistance than said terminal to limit thecurrent delivered to said heating element to a value below that required to sustain an are between said heating element and said sheath.

5. A sheathed electric heating element comprising a tubular metal sheath, a resistance wire heating element disposed within said sheath and electrically isolated therefrom, a pair of conductors of low electrical resistance forming terminals disposed within said sheath at a greater distance from said sheath than said heating element,

radially impervious solid insulation means for electrically isolating both of said terminals from said heating element and said sheath, and a pair of fusible elements respectively extending longitudinally from the ends of said terminals and connecting said terminals to said heating element, said fusible elements being electrically isolated from said sheath by solid insulation means for a substantial distance from the ends of said terminals and having an electrical resistance substantially equal to that of the heating element to limit the current passing from said terminals to said heating element to a value less than that required to sustain an arc between said heating element and said sheath.

References Cited in the file of this patent UNITED STATES PATENTS 

